1. Field of the Invention
This invention relates to a process and system for purifying waste liquid. More specifically, the invention is directed to a multi sorption process and system for treating such waste liquid.
2. Description of the Invention
For a great many years, liquids such as water have been treated to remove municipal and industrial wastes which have been deposited in them. In recent years, with a greater emphasis being placed on environmental protection, municipalities and industrial concerns have had to place an increased emphasis on obtaining an effluent having a high degree of purity from liquid waste removal systems.
Liquid waste removal processes and systems are concerned with two basic types of operations, one being the removal of settleable, floating and suspended solids from liquids and the other being the removal of dissolved materials from liquids. Settleable, floating and suspended solids, such as clays, bits of organic waste, and oil droplets, are removed by use of sedimentation tanks, flotation tanks, flocculation systems and screening. Dissolved material, including organic and inorganic compounds, are removed by processes such as reverse osmosis, electrodialysis, ion exchange, foam separation, chemical precipitation, chemical oxidation or acid leeching, distillation and adsorption on activated carbon or alumina. Biological processes are used to remove bio-degradable organic materials whether in suspended or dissolved form.
One biological purification system, generally known as the activated sludge process, is normally conducted as a continuous operation. A preliminary mechanical purification is usually affected to remove coarse suspended solids from the influent liquids. The liquid then passes into an aerating tank for biological purification. In the aerating tank, the liquid to be purified is aerated in the presence of micro- and macro-organisms for brevity referred to as biological or activated sludge. Due to the activity of the bacteria, the organic impurities present in the liquid are assimilated and decomposed and consequently the bacteria rapidly multiply. The added bacteria are removed by means of the macro-organisms, notably the bacteriophagic protozoae since the bacteria otherwise could only be removed with difficulty due to their lack of sedimentability. The liquid under treatment is then fed to a secondary purification tank in which the sludge is sedimented and the liquid under treatment leaves the secondary purification tank as biologically purified liquid. A part of the activated sludge sedimented in the secondary purification tank is recirculated to the aerating tank for reseeding and maintaining the desired sludge concentration in the aerating tank. The other part of the sludge is withdrawn and constitutes the sludge surplus.
One specific biological treatment process is disclosed in Dubach, U.S. Pat. No. 3,337,450 which provides for a first stage for allowing the bacteria to develop to their full efficiency and precipitate and a second process stage in which the bacteriophagic organisms can develop in preference to the bacteria to remove the balance of the bacteria.
The concentration of organic waste material in a liquid such as water may be measured in terms of biochemical oxygen demand (BOD) and chemical oxygen demand (COD) or other convenient terms. Each of these terms measures the amount of waste material in a sample of water in terms of the quantity of oxygen required to react with the waste material under prescribed conditions. BOD is a measure of the oxygen required by bacteria to degrade and decompose the waste material under prescribed conditions in a five-day period of time. COD is a measure of the equivalent amount of oxygen required for a strong oxidizing agent to oxidize the waste material to carbon dioxide and water under prescribed conditions. Both BOD and COD are expressed in milligrams per liter (mg/l).
Generally, industrial waste water and, in some cases, municipal waste water, contain impurities which poison or are toxic to or otherwise generally inhibit the growth of bacterial forms employed in the bacteriological treatment of waste water, which materials may sometimes be hereinafter referred to as bio-inhibiting contaminants. These bio-inhibiting contaminants include such materials as the toxics, phenols, nitro-phenols, pesticides and herbicides. When present in sufficient quantities these bio-inhibiting contaminants inhibit or "poison" the biological purification process described above, thereby decreasing the efficiency of the biological system, even to the point of sometimes "killing" the biological life.
Although these bio-inhibiting contaminants may be non-biodegradable and thus not considered to deplete the oxygen content of the receiving waters, they may biodegrade over a period greater than five (5) days measured by the BOD test and thus deplete oxygen in larger rivers and lakes. Furthermore, these bio-inhibiting contaminants may be noxious since such contaminants affect the taste, odor and color of the receiving waters and exhibit toxic effects on the fish and plant life therein. Thus, even when the biological treatment plants are operating under optimum conditions, the amount of organic contaminants removed may not be sufficient to meet present standards. As a consequence, there is a need for further treating of the effluents from such biological secondary treatment plants, as well as a need for an improved process for treating industrial waste waters in order to remove both biodegradable and non-biodegradable organic contaminants therefrom.
The use of various sorption materials to purify waste liquids, has been recognized by the prior art. For ease of description and without intending to limit this invention, the background of the invention will be further developed with particular reference to the sorptive material of activated carbon. In an adsorption process, the adsorbent activated carbon in this instance, attracts molecules of dissolved materials to it because of the pore structure of the carbon. Each granule or particle of the activated carbon contains a vast inter-connected network of pores of various sizes providing a large surface area for adsorbing molecules and thus has a large adsorption capacity. The molecules of the dissolved materials are lodged in the pores. These molecules are retained in the pores until they are removed, for example by thermal regeneration of the activated carbon which uses heat to release the previously dissolved molecules from the carbon. The prior art, in U.S. Pat. No. 3,803,029, has also shown a method of biologically regenerating adsorbent material for reuse.
It should be clearly understood that activated carbon in either a granular or powdered form is relatively expensive even though our activated carbon may be regenerated. Reactivation of the carbon is an expensive process. Furthermore, additional fresh activated carbon must be added to make up for the carbon lost in the reactivation process.
The prior art, in U.S. Pat. No. 3,855,123, has disclosed a sorption process in which the waste liquid passes through beds of granular carbon. The prior art has also disclosed passing industrial waste water through a bed of granular carbon before biologically treating that waste water. The BOD and COD levels of the influent, the water flowing into the system, and the BOD and COD levels of the effluent, the water flowing out of the system, are continuously measured. The system is designed to keep the BOD and/or COD level of the effluent below a prescribed amount. When the level of the BOD or the COD of the effluent rises above this amount, a "breakthrough" is said to occur in the sorbent material, in this case the activated carbon, must be replaced. This point of "breakthrough" is dependent on the concentration of the adsorbate, or contaminants adsorbed by the activated carbon. Activated carbon is capable of adsorbing a predetermined amount of contaminants depending on the concentration of those contaminants in the liquid and accordingly becomes saturated and incapable of adsorbing any more contaminants.
The activated carbon adsorbs dissolved molecules of the contaminants from water until it becomes saturated by remaining in contact with the water for a prescribed residence time. This time depends on the rate of adsorption of the adsorbent, for example the activated carbon, and the adsorption isotherm of the activated carbon. The "adsorption isotherm" is the relationship between the adsorptive capacity and equilibrium capacity or saturation point of the activated carbon adsorbent at a given constant temperature. It is expressed as a function of both the amount of adsorbate or contaminants which are adsorbed per unit weight of activated carbon and the concentration of the adsorbate in the water. For additional information concerning adsorption isotherms see U.S. Environmental Protection Agency Process Design Manual for Carbon Adsorption, Number EPA 625/1-71-002A, October, 1973, pages 4-4 through 4-7. It should be clearly understood that in either the granular carbon systems or the powdered carbon systems that the activated carbon adsorbs the dissolved molecules of the contaminants until breakthrough, at which time the activated carbon is saturated with the molecules of the contaminants.
In granular carbon systems, the waste liquid is purified by percolating it through either one or a number of columns containing activated carbon adsorbent. Where more than one column is used, the columns themselves are connected in series and parallel or in a series-parallel combination and the liquid may flow through them in an upflow or downflow direction or a combination thereof as disclosed in U.S. Pat. No. 3,855,123, the disclosure of which is hereby incorporated by reference. The size of the column or columns used is such that the liquid to be purified remains in contact with adsorbent for the prescribed residence time.
In powdered carbon systems, the liquid to be purified by the activated carbon is combined in a contacting container for the prescribed residence time and the carbon is separated from the purified liquid with a continuous filter or sedimentation operation.
The prior art has recognized in U.S. Pat. Nos. 3,904,518, 3,980,556 and 4,069,148 various processes for exposing waste water to activated sludge and a sorption material. Another prior art process is disclosed in U.S. Pat. No. 4,053,396 which provides for dissolving oxygen in the waste water while the waste water is passed through at least one bed of activated carbon. Other prior art methods, such as that disclosed in U.S. Pat. No. 4,008,161, provide a method for purifying waste water by passing an oxygen-containing gas through the waste water while the waste water is in intimate contact with finely divided activated carbon. A clarification agent is then added to the treated waste water to chemically precipitate the undesirable materials and the sludge thus formed from the treated water.
These prior art processes, methods and systems have limited effectiveness since their efficiency in removing contaminants from the waste water is not maximized. Due to the high cost of sorption material and systems, it is highly desirable to maximize their effectiveness and efficiency. The process and system of the present invention efficiently reduces the biological oxygen demand, the chemical oxygen demand and the total organic content of the waste water to provide an effluent liquid of improved purity. This process is particularly adaptable for treating liquids containing impurities which inhibit the biological treatment of the liquid, in particular industrial waste water alone or with mixtures of industrial waste water and sewage or other waste water.
It has been found that the present invention provides a process and system for treating a liquid containing impurities which inhibit the biological treatment of the liquid having an improved performance over such processes and systems of the prior art. The process of the present invention includes the steps of:
a. providing at least a first sorbent treatment zone and a second sorbent treatment zone;
b. treating the liquid in the first sorbent treatment zone with a sorbent material which has been previously used to treat the liquid in the second sorbent treatment zone;
c. passing the liquid through a biological treatment system;
d. treating the liquid received from said biological treatment system in the second sorbent treatment zone with a fresh activated sorbent material; and
e. transferring sorbent material from the second zone to the first zone after using the sorbent material in the second zone.
According to the adsorption isotherm, the amount of impurity or organic chemical adsorbed on the activated carbon is proportional to the concentration of the impurity in the solution. Thus, when the activated carbon is exposed to higher concentrations of impurities, the activated carbon will adsorb a greater amount of impurities.
For ease of description of the present invention, the term "fresh sorbent material" or "freshly activated sorbent material" or the like will include activated sorbent material, such as carbon, used in any process and activated sorbent material, such as carbon, which has previously been used and thereafter is regenerated or reactivated so that its sorptive capacity is substantially the same as the sorptive capacity of new or unused activated sorbent material.
By treating the liquid in the second sorbent treatment zone with "fresh" activated sorbent material, that is, sorbent material after activation or reactivation thereof and before use in the process to treat the liquid, the activated carbon will be exposed to waste water with relatively low concentrations of impurities. Consequently, the amount of the impurities actually adsorbed by the carbon is relatively small. Since the activated carbon is "fresh" the amount of impurities adsorbed thereby will be maximized and the effluent will be further purified.
Upon breakthrough of the sorbent material in the second sorbent treatment zone, the sorbent material is transferred to the first sorbent treatment zone. The first zone is used to treat the liquid having a higher concentration of impurities than the liquid treated by the second zone. The first zone is provided to move the impurities which inhibit the biological treatment of the liquid and may perform that function either before or during the biological treatment of the liquid. Since the sorbent material in the first zone is exposed to a liquid having a much higher impurity concentration than the liquid in the second zone, the sorbent material is able to adsorb a greater amount of impurities upon transfer from the second zone to the first zone. By removing biologically inhibitory contaminants in the first zone, the efficiency of the biological treatment and purity of the liquid treated thereby is substantially improved. This treatment of the liquid in the first zone is generally referred to as a "roughing" treatment. The system of the present invention also provides a "polishing" treatment of the liquid for removing impurities in the second zone which improves the effectiveness of the sorbent material. The process of this invention may be used to reduce the size of the treatment zones needed to treat a selected volume of waste water, thus lowering the initial plant costs. It increases the efficiency of the biological treatment over that of present biological treatment processes for a particular volume of waste water by lowering the treatment time and allowing higher flow rates. This invention also provides a process which removes colored impurities and bio-inhibiting contaminants while optimizing the effectiveness and efficiency of sorbent material. In addition to removing biologically inhibiting contaminants, the adsorbent also removes odors and other contaminants which cause foam from the waste water.